The complex populations of bacteria that colonize the oral cavity must be able to coordinate the metabolism of the wide variety of carbohydrates that are presented in salivary secretions and in the human diet. There is also intense competition between different species of bacteria for these limiting nutrients. In cases when dietary carbohydrates are provided in excess and the host develops dental caries, Streptococcus mutans and other acid-resistant species emerge as significant constituents of dental biofilms as a direct result of their capacity to grow and to continue to metabolize carbohydrates in acidified environments. Therefore, the persistence of the oral microbiome, the diversity of the oral microbiome, and the degree to which the oral microbiome is pathogenic are dictated by the genetic, biochemical and physiologic responses of these organisms to the source and availability of carbohydrates. Not surprisingly, then, the availability of metabolizable carbohydrates appears to be the single most important environmental factor affecting the composition and pathogenic potential of human dental plaque. The goals of this research are to provide a comprehensive understanding of the molecular basis for how the source and amount of carbohydrate modifies gene expression in the dental caries pathogen Streptococcus mutans. These studies utilize the most modern methods in molecular biology - coupled with the use of continuous-flow bioreactors to carefully control the physiology of the bacteria - to dissect how the source and the availability of carbohydrates modulate the pathogenic potential of the organisms.